Ok it depends who is building it - if its a specy builder - it will take longer because they are building so many at the one time yeah? If its not and just a "normal" builder - usually about 12 weeks? That would be the minimum, for a simple house etc.

My DH said its a great time to be building though, you can really get the prices down cos people are short of work. About the site - it depends on what the site is like, eg, lots of rocks in the dirt etc that they would have to dig up - how many trees etc....

ETA - and about the costs - quiz your builder, he should have an approximate idea since he's doing it all the time!